This invention generally relates to stress-engineered metal films, and more particularly to photo lithographically patterned spring structures formed from stress-engineered metal films.
Photo lithographically patterned spring structures have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. A typical spring structure includes a spring metal finger having an anchor portion secured to a substrate, and a free portion initially formed on a pad of release material. The spring metal finger is formed from a stress-engineered metal film (i.e., a metal film fabricated such that its lower portions have a higher internal tensile stress than its upper portions), such that the spring metal finger bends away from the substrate when the release material is etched away. The internal stress gradient is produced in the spring metal by layering different metals having the desired stress characteristics, or using a single metal by altering the fabrication parameters. Such spring metal structures may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, and actuated mirrors. Examples of such spring structures are disclosed in U.S. Pat. No. 3,842,189 (Southgate) and U.S. Pat. No. 5,613,861 (Smith).
The present inventors believe that many, if not most, potential commercial applications of spring structures will require metal plating on the free (released) portion of the spring metal finger. In some of these applications, the present inventors believe the spring metal structures will also require metal plating on the anchored portion of the spring metal finger. Accordingly, what is needed is a cost effective method for fabricating spring structures from stress-engineered metal film that include plated metal on the spring metal fingers.
The present invention is directed to efficient methods for fabricating spring structures in which a plated metal layer is formed on spring metal fingers after release from an underlying substrate. By plating the spring metal finger after release (i.e. after the finger is allowed to bend upward from the substrate due to internal stress), plated metal is formed on both the upper and lower surfaces of the spring metal finger simultaneously, thereby producing a low-cost spring structure exhibiting superior stiffness, thickness and electrical conductivity when compared to non-plated spring structures, or to spring structures plated before release.
In accordance with the disclosed fabrication method, a conductive release layer is deposited on a substrate, and then a stress-engineered (spring) metal film is formed on the release material layer. A first mask is then used to etch an elongated spring metal island from the metal film, but etching is stopped before the release layer is entirely removed to prevent undercutting that can cause premature release of the spring metal island. A release (second) mask is then deposited that defines a release window exposing a portion of the spring metal island and the release material layer surrounding this exposed portion. Subsequent removal of the release material layer exposed by the release mask causes the exposed portion of the spring metal island to bend away from the substrate due to its internal stress, thereby becoming the free portion of a spring metal finger, which also includes an anchored portion covered by the release mask. The release mask is then used to perform metal plating during which a plated metal layer is formed on the free portion of the spring metal finger, along with other selected structures exposed through the release mask.
In one embodiment, the plated metal is formed using an inexpensive electroplating procedure in which a conductive release layer is utilized as a cathode, thereby providing a thick spring structure that is significantly less expensive than spring structures having comparable thicknesses entirely formed by sputtering.
In another embodiment, the release mask, which is also used during the plating process, is provided with a channel extending over the anchored (i.e., non-released) portion of the spring metal finger, thereby facilitating the formation of plated metal on the anchor portion to improve conductivity.